Species identification based on a semi-diagnostic marker: Evaluation of a simple conchological test for distinguishing blue mussels Mytilus edulis L. and M. trossulus Gould

Abstract

Cryptic and hybridizing species may lack diagnostic taxonomic characters leaving researchers with semi-diagnostic ones. Identification based on such characters is probabilistic, the probability of correct identification depending on the species composition in a mixed population. Here we test the possibilities of applying a semi-diagnostic conchological character for distinguishing two cryptic species of blue mussels, Mytilus edulis and M. trossulus. These ecologically, stratigraphically and economically important molluscs co-occur and hybridize in many areas of the North Atlantic and the neighboring Arctic. Any cues for distinguishing them in sympatry without genotyping would save much research effort. Recently these species have been shown to statistically differ in the White Sea, where a simple character of the shell was used to distinguish two mussel morphotypes. In this paper, we analyzed the associations between morphotypes and species-specific genotypes based on an abundant material from the waters of the Kola Peninsula (White Sea, Barents Sea) and a more limited material from Norway, the Baltic Sea, Scotland and the Gulf of Maine. The performance of the “morphotype test” for species identification was formally evaluated using approaches from evidence-based medicine. Interspecific differences in the morphotype frequencies were ubiquitous and unidirectional, but their scale varied geographically (from 75% in the White Sea to 15% in the Baltic Sea). In addition, salinity-related variation of this character within M. edulis was revealed in the Arctic Barents Sea. For every studied region, we established relationships between the proportions of the morphotypes in the populations as well as between the proportions of the morphotypes in samples and the probabilities of mussels of different morphotypes being M. trossulus and M. edulis. We provide recommendations for the application of the morphotype test to mussels from unstudied contact zones and note that they may apply equally well to other taxa identified by semi-diagnostic traits.

Taxonomically mixed blue musselMytiluspopulations are spatially heterogeneous and temporally unstable in the subarctic Barents Sea

“…For mussels aged 4-7 years the shell morphotypes (E-morphotype, more characteristic of ME , or T-morphotype, more characteristic of MT ) were determined as in Khaitov et al, 2021. Only medium-aged mussels were used in the taxonomic analysis in order to avoid the bias due to a possible association between size and morphotypes in conspecific mussels (see Khaitov et al, 2021 for details).…”

“…1d and below), we clarified the relationship between Ptros and PT for local settlements. For this purpose, we have used 15 genotyped samples from the Tyuva Inlet (sample size 30-82 individuals, mean 44, STable 2 ), including nine from the published studies (Bufalova et al, 2005; Väinölä & Strelkov, 2011; Khaitov et al, 2021), stored in collections of the Department of Ichthyology and Hydrobiology (St. Petersburg State University), and six new samples collected in 2018 ( Fig. 1f, STable 1 ).…”

“…New samples were genotyped by the same set of allozyme loci “diagnostic” for the two species (Est-D, Gpi and Pgm) as in the studies listed above. Multilocus genotypes were classified into two categories, those dominated by ME genes and those dominated by MT genes, using Structure approach (Pritchard et al 2000) as in Khaitov et al, 2021. For ease of presentation, these categories will be referred to as “ M. trossulus ” ( MT ) and “ M. edulis ” ( ME ) although each could include hybrids in addition to purebreds (Khaitov et al, 2021).…”

“…It has recently been shown that ME and MT in the Murman populations differ by frequencies of shell morphotypes defined as absence or presence of an uninterrupted prismatic strip under the ligament on the inner side of the shell. In brackish localities (<30 ppt) the differences approach 65%, while in saline localities they make up only 18% (Khaitov et al, 2021). This means that in saline localities individual mussel assignment to one of the two species based on morphotypes is ineffective; however, the proportions of the species in samples can be predicted based on the morphotype frequencies fairly accurately (Khaitov et al, 2021).…”

“…In brackish localities (<30 ppt) the differences approach 65%, while in saline localities they make up only 18% (Khaitov et al, 2021). This means that in saline localities individual mussel assignment to one of the two species based on morphotypes is ineffective; however, the proportions of the species in samples can be predicted based on the morphotype frequencies fairly accurately (Khaitov et al, 2021).…”